Conventionally, a lighting device configured to power light-emitting diodes (LEDs) as a light source has been known. There is also a lighting device configured to power both a main light source formed of a large number of LEDs and an auxiliary light source which is formed of a small number of LEDs and configured to be driven as indirect light for example (e.g., JP Pub. No. 2011-222360).
FIG. 4 shows a circuit diagram of a conventional lighting device configured to power two light sources. The lighting device is formed of a full-wave rectifier circuit 101, a power-factor correction circuit 102, a first lighting power supply 103 and a second lighting power supply 104. The lighting device, of which input power supply is a commercial power supply E101 (100V, 50 Hz/60 Hz), is configured to supply lighting power to a first light source 105 as a main light source and a second light source 106 as an auxiliary light source.
The full-wave rectifier circuit 101 is formed of a diode bridge and configured to full-wave rectify an AC voltage Vac from the commercial power supply E101 to generate a rectified voltage Vdb.
The power-factor correction circuit 102 is formed of a step-up chopper circuit including an inductor L101, a diode D101, a switching device Q101 and a capacitor C101. A series circuit of the inductor L101 and the switching device Q101 is connected between output ends of the full-wave rectifier circuit 101. A series circuit of the diode D101 and the capacitor C101 is connected in parallel with the switching device Q101. The switching device Q101 is repeatedly turned on and off by switching control by a switch controller (not shown). The rectified voltage Vdb is consequently stepped up and a DC voltage Vdc11 is generated between both ends of the capacitor C101. Harmonic distortion of an input current from the commercial power supply E101 is reduced through the power-factor correction circuit 102 so that reduction in power factor is suppressed.
The first lighting power supply 103 is formed of a step-down chopper circuit including an inductor L102, a diode D102, a switching device Q102 and a capacitor C102. A series circuit of the capacitor C102, the inductor L102 and the switching device Q102 is connected between both output ends of the power-factor correction circuit 102. The diode D102 is connected in parallel with a series circuit of the capacitor C102 and the inductor L102. The switching device Q102 is repeatedly turned on and off by switching control by a switch controller (not shown). The DC voltage Vdc11 is consequently stepped down and a DC voltage Vdc12 is generated between both ends of the capacitor C102.
The first light source 105 is formed of light-emitting diodes LD101 in series, and connected in parallel with the capacitor C102. The DC voltage Vdc12 is to be applied thereto. The switch controller of the first lighting power supply 103 performs switching control of the switching device Q102 to control the DC voltage Vdc12 so as to supply a specified DC current to the first light source 105, thereby supplying an electric current to the first light source 105 by constant current control. The DC voltage Vdc12 is applied to the first light source 105 and the specified DC current flows therethrough, so that each light-emitting diode LD101 is lit.
The second lighting power supply 104 is formed of a step-down chopper circuit including an inductor L103, a diode D103, a switching device Q103 and a capacitor C103. A series circuit of the capacitor C103, the inductor L103, and the switching device Q103 is connected between output ends of the power-factor correction circuit 102. The diode D103 is connected in parallel with a series circuit of the capacitor C103 and the inductor L103. The switching device Q103 is repeatedly turned on and off by switching control by a switch controller (not shown). The DC voltage Vdc11 is consequently stepped down and a DC voltage Vdc13 is generated between both ends of the capacitor C103.
The second light source 106 is formed of light-emitting diodes LD101 in series, and connected in parallel with the capacitor C103. The DC voltage Vdc13 is to be applied thereto. The switch controller of the second lighting power supply 104 performs switching control of the switching device Q103 to control the DC voltage Vdc13 so as to supply a specified DC current to the second light source 106, thereby supplying an electric current to the second light source 106 by constant current control. The DC voltage Vdc13 is applied to the second light source 106 and the specified DC current flows therethrough, so that each light-emitting diode LD101 is lit.
Since the first light source 105 is employed as the main light source, it is formed of more light-emitting diodes LD101 than those of the second light source 106. Therefore, the DC voltage Vdc12 output from the first lighting power supply 103 is larger than the DC voltage Vdc13 output from the second lighting power supply 104. In the conventional example, the DC voltage Vdc12 is set to 300V, and the DC voltage Vdc13 is set to 20V.
Since the first and second lighting power supplies 103 and 104 are formed of respective step-down chopper circuits, the DC voltage Vdc11 output from the power-factor correction circuit 102 requires to be set higher than the DC voltages Vdc12 and Vdc13. In the conventional example, the DC voltage Vdc11 output from the power-factor correction circuit 102 is set to 410V.
Thus, in the conventional example, the first and second lighting power supplies 103 and 104 are connected in parallel with each other. The first lighting power supply 103, of which input power supply is the power-factor correction circuit 102, supplies lighting power to the first light source 105. The second lighting power supply 104, of which input power supply is the power-factor correction circuit 102, supplies lighting power to the second light source 106.
In the conventional example, the second lighting power supply 104 outputs the DC voltage Vdc13 of 20V, but it is low nevertheless, employs the DC voltage Vdc11 (=410V) output from the power-factor correction circuit 102 as an input power supply thereof. Therefore, the second lighting power supply 104 requires to be formed of components having a rated voltage (a withstand voltage) of 410V or more, which causes increase in size and cost of components constituting the second lighting power supply 104, and results in a problem of increase in size and costs of the lighting device per se.